Lee Broadway, a 41-year-old North Carolina mother of four, thought she was having a severe migraine. Ditto 49-year-old Lisa Colagrossi, a New York news reporter. Neither knew they were suffering from fatal aneurysms until they were rushed to the hospital.

A brain (or cerebral) aneurysm is a weakness in the wall of one of your brain’s blood vessels, says Dr Howard Riina, a neurosurgeon with New York University’s Langone Medical Center. As blood rushes through your brain, that weak point bulges and fills with blood. If it ruptures (“like a blow out on a tire,” says Riina), blood escapes into the surrounding brain tissue—a potentially deadly occurrence.

That doesn’t always happen; in fact around 6 million people—that’s about 1 in 50 Americans—may be unknowingly walking around with an unruptured aneurysm without realizing it, according to the Brain Aneurysm Foundation. And an estimated 80 percent of all aneurysms never rupture, the foundation says.

But the other 20 percent can be especially lethal. Up to 50 percent of patients die as soon as an aneurysm bursts, says Riina. Another third will survive but be left with severe cognitive disabilities. That’s scary, especially since most people don’t know how to ID if one is happening.

When the Lisa Colagrossi Foundation, a nonprofit organization devoted to aneurysm awareness and education, surveyed people about their aneurysm knowledge, not a single person could correctly ID the signs and symptoms that someone is having one.

If an aneurysm does rupture (and doctors aren’t sure what causes them to burst, though high blood pressure is one suspect), platelets in your blood will quickly plug the leak. But even a small amount of blood can increase pressure inside your head, says Riina.

“Patients often describe it as feeling like they’ve been struck by lightning,” says Riina. The brain ache can be in any part of your head—including behind your eyes or at the base of your skull—depending where the aneurysm is, he says. Often, the thudding is accompanied by neck stiffness, face tingling, sensitivity to light, dilated pupils, blurry vision, drooping eyelids, nausea or vomiting, or a sudden pain behind the eye. All are signs that the ruptured vessel or leaked blood are pushing on nearby nerves in your brain, says Riina.

To be clear, a stiff neck or eye pain on its own doesn’t mean you’ve got a brain bleed. But don’t blow off a single dilated pupil or suddenly droopy eyelid if they accompany—or come on the heels of—what Riina describes as “the worst headache of your life.” It’s critical to get to the ER or call 10177 stat so doctors can surgically relieve the pressure caused by your ruptured aneurysm.

Certain people have a higher risk for brain aneurysms, including people with a strong family history (more than one first-degree relative with brain aneurysm), obese individuals, smokers, and people who have certain conditions such as polycystic kidney disease. If you tick a few of these boxes, it’s worth talking to your family doctor to set up a preventative plan.

If an unrelated MRI or imaging scan (that you get, say, after a car accident, or to look for the cause of frequent migraines) happens across an aneurysm, don’t stress too much, as most doctors won’t treat it unless it’s above 5 mm in size, says Riina. That might sound terrifying, given how deadly the condition can be, but your doctor can monitor the aneurysm with a yearly brain scan to make sure the bulge isn’t growing in size. “If it does show signs that it’s getting bigger, we can go in and do surgery before it ruptures,” says Riina.

Looking for answers to other health problems? Here’s how you can tell if you have a stomach bug or if it’s just food poisoning, plus here’s what every woman needs to know about Toxic Shock Syndrome.